Chest drainage systems are in widespread use, and are available commercially from a number of manufacturers in the United States and other countries. Such systems include a thoracic catheter which is implanted in the chest cavity, and a water-sealed evacuation apparatus for removing air and liquid. The water-seal chamber communicates on one side with the catheter and on the other side with a source of controlled vacuum. In addition to the water-seal chamber, there may be provided a fluid collection chamber and a vacuum regulator chamber. If the vacuum is supplied through a pressure-regulator valve, the manometer chamber may be eliminated. Most commercial apparatus are at least "two bottle" systems, including a fluid collection chamber and a water-seal chamber. If they also include a pressure regulator chamber, they are commonly referred to as a "three-bottle system".
Application of a chest cavity evacuation system is required following: (1) surgical or traumatic opening of the thoracic cavity; or (2) life-threatening internal rupture of the lung tissue (pneumothorax) which may be spontaneous, or secondary to ventilator barotrauma (very common in premature infants), emphysema or infection. (Hydrothorax, chylothorax, and hemothorax are also indications for chest tube therapy.) Air and/or fluid in the pleural space is life-threatening and requires immediate placement of a pleural catheter with evacuation apparatus for the duration of the underlying problem.
In all of the foregoing applications, it is important to monitor the amount of air and/or fluid being removed from the chest cavity as an indicator of the course of the underlying process and need of continued therapy. When a fluid collection chamber is provided, it can be calibrated so that the amount of removed fluid can be visually determined. Accurate metering of the removed air, however, presents a more difficult problem. The air evacuated bubbles through the water-seal, and therefore a qualitative observation that air is being removed can be visually confirmed. But heretofore there has been no satisfactory metering device for quantitatively measuring the volume of removed air.
U.S. Pat. No. 3,683,913 discloses a chest evacuator having an air-flow meter associated with a water-seal chamber. By utilizing a series of air-flow passages progressively increasing in diameter away from the position of air entry into the water-seal, observation of the number of chambers in which the air is passing out of the water-seal can be used to give a rough estimate of the air flow rate. Chest drainage apparatus incorporating this kind of flow-rate meter has been marketed commercially in the United States. At high air flow rates, by determining and recording the estimated flow rates at the start and end of an observation period, the approximate total volume of evacuated air can be calculated. This kind of flow rate metering, however, is not defined at low flow rates, such as are encountered near the critical time for removal of the chest drainage unit, or during most of the time of use of a pediatric/neonatal drainage unit.
What has been needed is a convenient low volume meter which can be used to accurately determine low volume flows. As the volume of removed air decreases and approaches zero, it is difficult to observe even the small amount of bubbling through the water-seal, or to estimate the rate of flow by a metering device like the one described in U.S. Pat. No. 3,683,913.
It is important to determine the end point of use of the chest evacuation apparatus. When no more air is being removed from the chest cavity, this indicates that the desired negative pressure condition has been restored in the chest cavity. At flow rates of a few milliliters per hour, bubbling in the water-seal is infrequent (e.g., one bubble every few minutes). The attending nurse may need to spend an undue amount of time trying to observe the operation of the drainage unit and frequently records inaccurate observations. However, heretofore no metering device has been provided which permits the nurse to make inspections at intervals of several hours even though the conclusion of the air removal is approximate.
If it is erroneously concluded that no more air is being removed, whereas a small volume flow is actually occurring, this can lead to premature discontinuance of the use of the chest cavity evacuation apparatus with removal of the catheter. Such premature removal, when later discovered, can result in an acute emergency (tension pneumothorax), and will require reinsertion of the catheter and re-establishment of the chest cavity drainage. This is a costly and potentially fatal hospital situation which is very desirable to avoid.
Further, once the fact has been confirmed that no air is being removed from the chest cavity, it is imperative to remove the catheter and discontinue use of the chest evacuation apparatus in a short time, usually within 12 to 24 hours. The risk of chest catheter infection increases with duration of placement, necessitating prompt removal of the catheter upon resolution of the underlying problem. Heretofore, however, the available chest drainage units have sometimes been left connected for longer times than required in order to ascertain that air removal had concluded, thus placing the patient at higher risk of morbidity and mortality from chest catheter complications.